1. Field of the Invention
The present invention relates to an electronic pen, an electronic whiteboard system, and a projector system, and in particular, relates to a mechanism of an electronic pen for transmitting an ultrasonic wave.
2. Description of the Related Art
An electronic whiteboard system using an ultrasonic wave and an infrared ray is widely used. FIG. 1 is a schematic diagram illustrating the configuration of a conventional electronic whiteboard system. Electronic whiteboard system 101 includes electronic pen 102, ultrasonic wave sensors 3a, 3b, infrared ray sensors 4a, 4b, and coordinate calculator 5 that is connected to ultrasonic wave sensors 3a, 3b and infrared ray sensors 4a, 4b. Ultrasonic wave sensors 3a, 3b and infrared ray sensors 4a, 4b are provided on writing surface S such as a screen, a wall, or a whiteboard. Ultrasonic wave sensor 3a and infrared ray sensor 4a are arranged in pairs adjacent to each other at corner 6a of writing surface S. Similarly, ultrasonic wave sensor 3b and infrared ray sensor 4b are arranged in pairs adjacent to each other at corner 6b. 
FIG. 2 is a schematic view of a conventional electronic pen. Electronic pen 102 is a pen-shaped transmission device that is provided with elongate housing 20 having longitudinal axis C. Ultrasonic wave transmitter 121 and infrared ray transmitter 122 are arranged near write side tip 124 of housing 20 and transmit an ultrasonic Wave and an infrared ray in a concentric pattern, respectively. Therefore, ultrasonic wave sensors 3a, 3b and infrared ray sensors 4a, 4b can receive an ultrasonic wave and an infrared ray while a user holds electronic pen 102 in a usual manner without paying attention to the orientation of electronic pen 102 around longitudinal axis C. The configuration of such an electronic pen is disclosed in Japanese Patent Laid-Open Publication No. 2004-192199 and No. 237950/99. Further, Japanese Patent Laid-Open Publication No. 203043/99 discloses an electronic pen having ultrasonic wave transmitters that are provided at the same intervals around the longitudinal axis thereof. In the present specification, the term “orientation” indicates an angle or a rotational orientation of an electronic pen about the longitudinal axis thereof.
When the tip of electronic pen 102 is pressed against writing surface S, an ultrasonic wave and an infrared ray are simultaneously transmitted from ultrasonic wave transmitter 121 and infrared ray transmitter 122, respectively. FIG. 3 is a cross-sectional view taken along line 3-3 in FIG. 2. An ultrasonic wave and an infrared ray travel in a concentric circle pattern from the tip of electronic pen 102, as shown by the dashed lines in FIG. 3. Ultrasonic wave sensors 3a, 3b receive the ultrasonic wave, which is shown by the dashed lines in FIG. 1 and are extended from the tip of electronic pen 102. Infrared ray sensors 4a, 4b receive an infrared ray, which is shown by the chain lines FIG. 1 and are extended from the tip of electronic pen 102. FIG. 4 is a conceptual diagram explaining the difference in the arriving time of an infrared ray and an ultrasonic wave. Infrared ray, which is light, instantaneously reaches infrared ray sensor 4a (or 4b). On the other hand, an ultrasonic wave, which is an acoustic wave and travels slower than an infrared ray, reaches ultrasonic wave sensor 3a (or 3b) after the infrared ray reaches infrared ray sensor 4a. Coordinate calculator 5 measures time lag t1, which is the difference between the time at which the ultrasonic wave is received and the time at which the infrared ray is received, and calculates distance D1 (see FIG. 1) between corner 6a and (the tip of) electronic pen 102 based on time lag t1. The same process is carried out by infrared ray sensor 3b and ultrasonic wave sensor 4b to calculate distance D2 (see FIG. 1) between corner 6b and (the end of) electronic pen 102. Coordinate calculator 5 determines the coordinate of electronic pen 102 relative to a predetermined reference point based on the principle of triangulation by using distances D1, D2 that are calculated. Since the ultrasonic wave and the infrared ray are transmitted intermittently at a regular interval, as shown in FIG. 4, textual or graphic information, which is written on the writing surface by electronic pen 102, can be reproduced as electronic data by connecting the coordinates that have been determined.
In recent years, an electronic whiteboard system, or a projector system that is provided with an ultra-short focal length projector having a mirror optical system, as shown in FIGS. 5A and 5B, has been developed. FIG. 5A is a schematic diagram of a projector system viewed from the front of a screen, and FIG. 5B is a schematic diagram viewed from the side of the screen. Projector 67 has infrared ray sensor 65 and ultrasonic wave sensors 66a, 66b. Projector 67 also has a coordinate calculator, not shown, to calculate three-dimensionally the distance between electronic pen 62 on screen 61 and projector 67, based on the difference between the time at which infrared ray pulse 63, which is transmitted from electronic pen 62, is received by infrared ray sensor 65 and the time at which ultrasonic wave pulses 64a, 64b are received by ultrasonic wave sensors 66a, 66b, and on the distance between projector 67 and screen 61, which is measured by projector 67. Since infrared ray sensor 65 and ultrasonic wave sensors 66a, 66b are arranged on projector 67 adjacent to each other, which is different from the example in FIG. 1, a single infrared ray sensor 65 may be enough to provide a reference time to measure the difference between the arriving time of the infrared ray and the arriving time of the ultrasonic wave. An electronic whiteboard system of this type has the advantage that no screen is needed, and that a wall etc. in a room can be used, if practical, because ultrasonic wave sensors and an infrared ray sensor are provided in a projector. Further, no means is required to connect the ultrasonic wave sensor/infrared ray sensor to a personal computer. See Japanese Patent Laid-Open Publication No. 2005-115870 for details of such an electronic whiteboard system.
However, since the ultrasonic wave that is transmitted from the ultrasonic wave transmitter travels concentrically, actually in three dimensions, there is the possibility that the ultrasonic wave that is transmitted in directions other than the direction of the ultrasonic wave sensor is reflected by an object and reaches the ultrasonic wave sensor as a reflected wave. As a result, an electronic whiteboard system, which detects the coordinate of a pen under the condition that an ultrasonic wave directly reaches the ultrasonic wave sensor from the ultrasonic wave transmitter along a straight line, cannot detect the coordinate accurately due to the reflected wave. For example, if an ultrasonic wave that is reflected reaches the ultrasonic wave sensor with delay, and, at the same time, the next ultrasonic wave that is transmitted later reaches the ultrasonic wave sensor along a straight line, then there is the possibility that an ultrasonic wave pulse that is to be detected cannot be recognized correctly. A similar problem may also occur when a pulse of ultrasonic wave reaches the ultrasonic wave sensor along a straight line, then the ultrasonic wave that is reflected reaches the ultrasonic wave sensor, and then the next pulse reaches the ultrasonic wave sensor along a straight line. In this case, the reflected wave is erroneously recognized as the next pulse of the ultrasonic wave that will reach the ultrasonic wave sensor. In a conventional electronic whiteboard system, as shown in FIG. 1, the influence of a reflected wave can be reduced by attaching a canopy to the ultrasonic wave sensor to provide the ultrasonic wave sensor with directivity, because ultrasonic wave sensor is provided on the approximately same plane as the screen. However, a canopy will increase the size of the ultrasonic wave sensor, as well as the cost. On the other hand, in an electronic whiteboard system or a projector system using an ultra-short focal length projector, as shown in FIG. 5, it is difficult to provide an ultrasonic wave sensor with directivity by using a canopy, because the ultrasonic wave sensor is arranged away from the screen, or the projection surface. In an electronic whiteboard system, it is difficult to provide an ultrasonic wave sensor with directivity for a specific direction, because the distance between the projector and the projection surface differs depending on the size of the image that is to be projected.